Conventionally, a method for adjusting the humidity of at least one of two gases having different humidity by disposing the two gases so as to be adjacent to each other via a filter medium to which moisture is permeable (moisture permeable filter medium), thereby moving the moisture (water vapor) from the gas having higher humidity to the gas having lower humidity via the moisture permeable filter medium has been known. For example, a method using a member constituted by housing an adsorbent in a bag that is formed using a moisture permeable filter medium is known (see Patent Literature 1).
The air inside the member has lower humidity than the air outside the bag because the adsorbent adsorbs water vapor. This allows the water vapor in the outside air to permeate through the moisture permeable filter medium so as to enter the bag, so that the water vapor is adsorbed by the adsorbent. That is, humidity adjustment (humidification) of the air (the air outside the bag) in a specific space is enabled by disposing such a member in the space.
The moisture permeable filter medium as described above needs to allow water vapor to permeate therethrough and to be capable of partitioning a space well into spaces that are adjacent to each other via the moisture permeable filter medium. As a material constituting such a moisture permeable filter medium, a polytetrafluoroethylene porous film (hereinafter, referred to also as a PTFE porous film) is used. Such a PTFE porous film is known to have good moisture permeability. Therefore, by disposing the two gases having different humidity so as to be adjacent to each other via the PTFE porous film, the movement of the water vapor (by permeating through the moisture permeable filter medium) from the gas having higher humidity to the gas having lower humidity is enabled, while the two gases are separated.
Meanwhile, the PTFE porous film as described above is comparatively thin and is not stiff. Therefore, sole use of the PTFE porous film as a moisture permeable filter medium results in poor handling. Therefore, a moisture permeable filter medium having good handleability (stiffness) is provided by laminating an air permeable sheet having a certain thickness to the PTFE porous film.
However, such a moisture permeable filter medium is thicker than the PTFE porous film alone, because the air permeable sheet is laminated to the PTFE porous film. Here, the moisture permeability to the moisture permeable filter medium (mobility of the water vapor from one surface side to the other surface side) is inversely proportional to the thickness of the filter medium. Therefore, the moisture permeability to the aforementioned moisture permeable filter medium is lower than that to the PTFE porous film alone. Thus, the water vapor cannot effectively permeate through the moisture permeable filter medium from one surface side to the other surface side.